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    Subject[PATCH 22/47] writeback: prevent bandwidth calculation overflow
    On 32bit kernel, bdi->write_bandwidth can express at most 4GB/s.

    However the current calculation code can overflow when disk bandwidth
    reaches 800MB/s. Fix it by using "long long" and div64_u64() in the
    calculations.

    And further change its unit from bytes/second to pages/second.
    That allows up to 16TB/s bandwidth in 32bit kernel.

    CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Acked-by: Rik van Riel <riel@redhat.com>
    Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
    ---
    include/linux/backing-dev.h | 5 +++--
    mm/backing-dev.c | 4 ++--
    mm/page-writeback.c | 14 +++++++-------
    3 files changed, 12 insertions(+), 11 deletions(-)

    --- linux-next.orig/mm/page-writeback.c 2010-12-08 22:44:28.000000000 +0800
    +++ linux-next/mm/page-writeback.c 2010-12-08 22:44:29.000000000 +0800
    @@ -531,7 +531,7 @@ void bdi_update_write_bandwidth(struct b
    unsigned long written;
    unsigned long elapsed;
    unsigned long bw;
    - unsigned long w;
    + unsigned long long w;

    if (*bw_written == 0)
    goto snapshot;
    @@ -550,9 +550,10 @@ void bdi_update_write_bandwidth(struct b
    goto snapshot;

    written = percpu_counter_read(&bdi->bdi_stat[BDI_WRITTEN]) - *bw_written;
    - bw = (HZ * PAGE_CACHE_SIZE * written + elapsed/2) / elapsed;
    + bw = (HZ * written + elapsed / 2) / elapsed;
    w = min(elapsed / unit_time, 128UL);
    - bdi->write_bandwidth = (bdi->write_bandwidth * (1024-w) + bw * w) >> 10;
    + bdi->write_bandwidth = (bdi->write_bandwidth * (1024-w) +
    + bw * w + 1023) >> 10;
    bdi->write_bandwidth_update_time = jiffies;
    snapshot:
    *bw_written = percpu_counter_read(&bdi->bdi_stat[BDI_WRITTEN]);
    @@ -577,7 +578,7 @@ static void balance_dirty_pages(struct a
    unsigned long dirty_thresh;
    unsigned long bdi_thresh;
    unsigned long task_thresh;
    - unsigned long bw;
    + unsigned long long bw;
    unsigned long pause = 0;
    bool dirty_exceeded = false;
    struct backing_dev_info *bdi = mapping->backing_dev_info;
    @@ -640,8 +641,7 @@ static void balance_dirty_pages(struct a
    * of dirty pages have been cleaned during our pause time.
    */
    if (nr_dirty < dirty_thresh &&
    - bdi_prev_dirty - bdi_dirty >
    - bdi->write_bandwidth >> (PAGE_CACHE_SHIFT + 2))
    + bdi_prev_dirty - bdi_dirty > (long)bdi->write_bandwidth / 4)
    break;
    bdi_prev_dirty = bdi_dirty;

    @@ -664,7 +664,7 @@ static void balance_dirty_pages(struct a
    bw = bw * (task_thresh - bdi_dirty);
    do_div(bw, bdi_thresh / TASK_SOFT_DIRTY_LIMIT + 1);

    - pause = HZ * (pages_dirtied << PAGE_CACHE_SHIFT) / (bw + 1);
    + pause = HZ * pages_dirtied / ((unsigned long)bw + 1);
    pause = clamp_val(pause, 1, HZ/10);

    pause:
    --- linux-next.orig/mm/backing-dev.c 2010-12-08 22:44:24.000000000 +0800
    +++ linux-next/mm/backing-dev.c 2010-12-08 22:44:29.000000000 +0800
    @@ -103,7 +103,7 @@ static int bdi_debug_stats_show(struct s
    (unsigned long) K(bdi_stat(bdi, BDI_RECLAIMABLE)),
    K(bdi_thresh), K(dirty_thresh), K(background_thresh),
    (unsigned long) K(bdi_stat(bdi, BDI_WRITTEN)),
    - (unsigned long) bdi->write_bandwidth >> 10,
    + (unsigned long) K(bdi->write_bandwidth),
    nr_dirty, nr_io, nr_more_io,
    !list_empty(&bdi->bdi_list), bdi->state);
    #undef K
    @@ -662,7 +662,7 @@ int bdi_init(struct backing_dev_info *bd
    goto err;
    }

    - bdi->write_bandwidth = 100 << 20;
    + bdi->write_bandwidth = (100 << 20) / PAGE_CACHE_SIZE;
    bdi->dirty_exceeded = 0;
    err = prop_local_init_percpu(&bdi->completions);

    --- linux-next.orig/include/linux/backing-dev.h 2010-12-08 22:44:24.000000000 +0800
    +++ linux-next/include/linux/backing-dev.h 2010-12-08 22:44:29.000000000 +0800
    @@ -74,9 +74,10 @@ struct backing_dev_info {

    struct percpu_counter bdi_stat[NR_BDI_STAT_ITEMS];

    - struct prop_local_percpu completions;
    + unsigned long write_bandwidth;
    unsigned long write_bandwidth_update_time;
    - int write_bandwidth;
    +
    + struct prop_local_percpu completions;
    int dirty_exceeded;

    unsigned int min_ratio;



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